Zinc in the proper amounts plays a crucial role in neurological function. However, too much zinc can be as harmful as too little. On the one hand, appropriate levels of zinc supplementation can improve mental function in children who are likely to have low levels of zinc. On the other hand, too much zinc has been reliably linked to mild mental impairments and cell death.
Zinc and Cognition
In a study published in the 2009 issue of the "Journal of the American College of Nutrition," adolescent girls in India were either given zinc-rich snacks that had been prepared in ways to make the mineral more bioavailable, or they were given an ayurvedic preparation of zinc known as jasad bhasma. Jasad bhasma is zinc that has been reduced to ash and then blended with herbs.
The results of the study showed that girls given zinc-rich snacks or jasad bhasma had higher scores on a test of visual memory than did girls in the control group. They also had faster simple reaction and recognition reaction times.
It is important to note that the girls received zinc as part of foods rather than as pills, a situation that differs from standard western experiments involving zinc and that may have influenced the results.
Animal Research on Zinc
When rats are injected with clioquinol, a substance that reduces the amount of zinc available in the brain, these animals perform normally on recognition tasks that they are presented with one hour after training. However, according to a December 2010 article in "Neuroscience," if they are presented with this task 24 hours after training, their performance is impaired compared to animals who have not received the drug. The researchers concluded that inhibition of zinc release in the brain impairs the formation of long-term memory by the hippocampus.
On the other hand, zinc levels that are too high can also impair memory. A study published in "Physiology & Behavior" in May 2011 reports that rats given zinc in their drinking water performed more poorly on a test of spatial memory than did rats given water only. Yet when both copper and zinc were added to their drinking water, they performed as well as controls.
The research on animals, taken together, suggests that while zinc is necessary for memory formation, zinc and copper must be present in proper ratios to ensure proper neurological functioning.
Alzheimer's Disease
The role played by zinc in Alzheimer's disease has been the subject of vigorous controversy for more than a decade. On the one hand, mice given supplemental zinc showed spatial memory impairments that are similar to those experienced by humans with the disease. On the other hand, a meta-analysis of studies on brain iron, zinc and copper levels published in the May 2011 issue of "Progress in Neurobiology" found that zinc levels in the cortices of Alzheimer's brains were not significantly different, but copper levels were depleted.
Memory Loss After Heart Attacks
During a heart attack, the brain is often deprived of oxygen, leading to neuronal death and side effects like memory loss. According to a press release first issued by the National Institute of Neurological Disorders and Stroke in 1996 and updated in 2009, zinc contributes to cell death. When this mineral enters oxygen-deprived cells, they die. Neuronal death can often be prevented by injecting heart attack patients with a substance that reduces the amount of zinc between neurons. As Thomas Jacobs, Ph.D., commented, "In many cases, the substances that kill neurons and other cells are the ones we need in lesser amounts."
While the data on zinc and memory are difficult to untangle, it does seem that the zinc to copper ratio is as important as the amount of zinc alone. Because too much zinc can have such serious consequences, anyone taking zinc supplements would be wise to consult a physician.
References
- "Neuroscience"; Impairment of Recognition Memory and Hippocampal Long-term Potentiation After Acute Exposure to Clioquinol; A. Takeda, et al.; December 1, 2010
- "Physiology & Behavior"; Alterations in Fear Response and Spatial Memory in Pre- and Post-natal Zinc Supplemented Rats: Remediation by Copper; A.M. Railey, et al.; May 2011
- National Institute of Neurological Disorders and Stroke: Study Implicates Zinc in Neuron Loss After Heart Attack
- "Journal of Alzheimer's Disease"; The Effects of Enhanced Zinc on Spatial Memory and Plaque Formation in Transgenic Mice; D.H. Linkous, et al.; 2009
- "Progress in Neurobiology"; Iron, Zinc and Copper In the Alzheimer's Disease Brain: A Quantitative Meta-analysis. Some Insight On the Influence of Citation Bias on Scientific Opinion; M. Schrag, et al.; May 2011
- "Journal of the American College of Nutrition"; Zinc Supplementation Improved Cognitive Performance and Taste Acuity In Indian Adolescent Girls; R.P. Tupe, et al.; August 2009
